Ethylene glycol and propylene glycol are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers, such as PET. Historically, ethylene and propylene glycols have been made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, produced from fossil fuels.
More recently, efforts have focused on producing chemicals, including glycols, from renewable feedstocks, such as sugar-based materials. The conversion of sugars to glycols offers an efficient use of the starting materials with the oxygen atoms remaining intact in the desired product.
Current methods for the conversion of saccharides to sugars revolve around a hydrogenation/hydrogenolysis process as described in Angew. Chem. Int. Ed. 2008, 47, 8510-8513.
WO 2015/028398 describes a continuous process for the conversion of a saccharide-containing feedstock into glycols. In this process the saccharide-containing feedstock is contacted in a reactor with a catalyst composition comprising at least two active catalytic components comprising, as a first active catalyst component, one or more materials selected from transition metals from groups 8, 9 or 10 or compounds thereof, with catalytic hydrogenation capabilities; and, as a second active catalyst component, one or more materials selected from tungsten, molybdenum and compounds and complexes thereof. The second active catalyst component may be present in homogeneous form.
Conversion of saccharides to glycols in the presence of a retro-aldol catalyst and a hydrogenation catalyst is highly sensitive to pH. In particular, in the absence of pH control, the pH of the reaction system decreases uncontrollably due to the formation of organic acids. Where the retro-aldol catalyst is a tungstate, the decrease in pH results in precipitation of the tungstate which is therefore detrimental to the process. In order to avoid or reduce such precipitation of catalyst, it is known to add one or more acids to buffer the reaction system.
WO 2015/154258 describes a process for converting saccharide-containing feedstock into ethylene glycol by contacting the feedstock with a two component catalyst system in the presence of hydrogen at a pH of from 2.0 to 6.5. The process is conducted in the presence of an organic or inorganic acid.
The present inventor has found that acids introduced into the process to act as buffers for maintaining the pH in the desired range are found in the water and glycol streams that are separated from the reactor product stream. Such contamination of the water and glycol product streams is undesirable since it necessitates additional processing to remove the acids from the product streams and further addition of acids to the reactor to maintain the desired buffering effect, hence increasing the cost of the glycol production.
It would therefore be advantageous to provide an improved process for pH control in the conversion of saccharides to glycols.